This invention is concerned with wastewater treatment, and particularly a sewage treatment plant including membrane separators, in a system that can handle up to three times or more normal flow during peak flow conditions, such as storm conditions in a combined sewer system. The invention enables the use of membrane separators for both a storm event and for sludge stabilization during normal flow, with minimum operator intervention.
Membrane separators, sometimes called membrane bioreactors or MBRs, have come into increasing use in recent years, particularly in small to medium sized sewage treatment plants, as an efficient means of removing water and thickening sludge in a way that conserves space and capital cost. MBRs, however, are limited in terms of peak flow conditions, due to their inability to handle high peak flow conditions, normally no more than twice design flow. To overcome this limitation, either additional membranes must be utilized for peak flows, or flows in excess of twice design flow must be stored, in a separate basin, or bypassed.
Membrane separator systems in wastewater treatment processes are shown in U.S. Pat. Nos. 5,192,456, 6,843,908 and 6,277,209. Further, the assignee of this invention has developed a pre-thickened aerobic digestion (PAD) process using membrane thickeners in the context of sludge digestion. In one form this process incorporates membranes and an aerobic/anoxic configuration to allow simultaneous nitrification and denitrification to occur while the sludge is being thickened. See U.S. Pat. Nos. 6,712,970 and 6,743,362.
Until the current invention, membrane separators for thickening wastewater were designed essentially as a completely separate process. The MBRs in such a system were required to be designed to handle peak flow or storm conditions which could be up to three times normal flow, requiring much excess capacity and cost as regards normal flow.